Chapter6
Absorption and Stripping of
Dilute Mixtures
Exercises
Exercise6.1
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Prior to 1950, only two types of commercial random
packings were in common use: Raschig rings and
Berl saddles. Starting in the 1950s, a wide variety of
commercial random packings began to appear. What
advantages do these newer packings have? By what
advances in packing design and fabrication
techniques were these advantages achieved? Why
were structured packings introduced?
Exercise6.2
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In Example 6.3, a lean oil of 250 MW is used as the
absorbent. Consideration is being given to the
selection of a new absorbent. Available streams are:
Rate, Density,
gpm
1b/gal
MW
C5s
115
5.24
72
Light oil
36
6.0
130
Medium oil
215
6.2
180
Which stream would you choose? Why? Which
streams, if any, are unacceptable?
Exercise6.3
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A straw oil used to absorb benzene from coke-oven gas is to be
steam-stripped in a sieve-plate column at atmospheric pressure
to recover the dissolved benzene. Equilibrium conditions at the
operating temperature are approximated by Henry's law such
that, when the oil phase contains 10 mol% CfiHft, the CeHfi
partial pressure above the oil is 5.07 kPa. The oil may be
considered nonvolatile. The oil enters containing 8mol%
benzene, 75% of which is to be recovered. The steam leaving
contains 3 mol% C6H6. (a) How many theoretical stages are
required? (b) How many moles of steam are required per 100
mol of oil-benzene mixture? (c) If 85% of the benzene is to be
recovered with the same oil and steam rates, how many
theoretical stages are required?
Exercise6.4
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One thousand kilomoles per hour of rich gas at
70°F with 25% C1, 15% C2, 25% C3, 20% nC4,
and 15% nC5 by moles is to be absorbed by
500kmol/h of nC10 at 90°F in an absorber
operating at 4atm. Calculate by the Kremser method
the percent absorption of each component for four,
ten, and thirty theoretical stages. What do you
conclude from the results? (Note: The K-value of
nC10 at 80°F and 4atm is 0.0014.)
Exercise6.5
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The absorption operation of Examples 6.1 and 6.4 is
being scaled up by a factor of 15, such that a column
with an 11.5-ft diameter will be needed. In addition,
because of the low efficiency of 30% for the original
operation, a new tray design has been developed
and tested in an Oldershawtype column .The
resulting Murphree vapor-point efficiency, EOV, for
the new tray design for the system of interest is
estimated to be 55%. Estimate EMV and E0. (To
estimate the length of the liquid flow path, ZL, use
Figure 6.16. Also. assume that u/de = 6
Exercise6.6
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Conditions at the bottom tray of a reboiled
stripper are as shown in Figure 6.48. If valve
trays are used with a 24-in. tray spacing,
estimate the required column diameter for
operation at 80% of flooding.
Exercise6.7
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A wastewater stream of 600gpm, containing 10ppm (by weight)
of benzene, is to be stripped with air in a packed column
operating at 25°C and 2atm to produce water containing
0.005ppm of benzene. The packing is 2-in. Flexirings made of
polypropylene. The vapor pressure of benzene at 25°C is
95.2torr. The solubility of benzene in water at 25°C is 0.180
g/100 g. An expert in VOC stripping with air has suggested use
of 1,000scfm of air (60°F, 1atm), at which condition one should
achieve for the mass transfer of benzene:
kLa=0.067s-1
and
kga=0.80s-1
Determine:
(a) The minimum air stripping rate in scfm. Is it less than the
rate suggested by the expert? If not, use 1.4 times your
minimum value.
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(b) The stripping factor based on the air rate
suggested by the expert.
(c) The number of transfer units, NOG
required.
(d) The overall mass transfer coefficient, KGa,
in units of mol/m3-s-kPa and s-1. Which
phase controls mass transfer?
(e) The volume of packing in cubic meters.